I dunno...JHMO but if you finish bodywork it, blocking and all, and then swap frames and the gaps are off you may end up redoing some stuff. Worse yet if you painted it and then did the frame swap?
Just me, but I'd set it up with the new frame under it first OR do a limited amount of body work and then final when the new frame is under it.
And Don makes a great point above.
Hope this helps...
